Semiconductor devices are typically constructed en masse on a silicon or gallium arsenide wafer through a process which comprises a number of deposition, masking, diffusion, etching and implanting steps. When the devices are sawed into individual units, each takes the form of an integrated circuit (IC) die. Each die is, essentially, a wafer-thin rectangular solid, having two pair of substantially planarly parallel edges, all edges being of substantially uniform width since the die was fabricated on a wafer of substantially uniform thickness throughout, and each pair of parallel edges being perpendicular to the other pair. Each die has a lower surface (also referred to herein as the back of the die) that is devoid of circuitry, and an upper surface (also referred to herein as the face of the die) with integrated circuitry constructed thereon, the circuitry being electrically accessible via die wire bonding pads which may be arranged in a variety of configurations on the face or edges of the die. In order to interface a die with other circuitry, it is (using contemporary conventional packaging technology) mounted on the die-mounting paddle of a leadframe strip which consists of a series of interconnected leadframes. The die-mounting paddle of a standard leadframe is larger than the die itself, and it is surrounded by the wire bonding pads of individual leads. The input/output wire bonding pads of the die are then connected one by one in a wire-bonding operation to the leadframe's wire bonding pads with extremely fine gold or aluminum wire. Following the application of a polyimide protective layer to the face of the die, it, and a portion of the leadframe to which it is attached, is encapsulated in a plastic material, as are all other die-leadframe assemblies on the leadframe strip. A trim and form operation then separates the resultant interconnected packages and bends the leads of each package into the proper configuration.
FIG. 1 is a cut-away isometric view of a typical conventional small-outline J-lead memory package, showing a die 11 bonded to the die-bonding paddle 12 of a standard-design leadframe 13, with each of the die's wire-bonding pads 14 connected to an associated leadframe wire-bonding pad 15 by means of fine metal wires 16.
In the interest of higher performance equipment and lower cost, increased miniaturization of components and greater packaging density have long been the goals of the computer industry. IC package density is primarily limited by the area available for die mounting.
A conventionally-designed leadframe resists attempts to increase circuit density. Since the die-bonding paddle must be larger than the die which is bonded to it, the larger the die, the less space that remains around the periphery of the die-bonding pad for wire bonding. In addition, the wire-bonding pads on the standard leadframe provide anchorage for the leads when they and the die are encapsulated in plastic. Therefore, as the die size is increased in relation to package size, there is a corresponding reduction in the space along the sides of the package for the encapsulating plastic which joins the top and bottom plastic body halves at the mold part line and anchors the leads. As the leads are subjected to the normal stresses of forming and assembly operations, the plastic may crack, with a consequent destruction of the package seal and a substantial increase in the probability of premature package failure. For these reasons, the standard leadframe design is not particularly adaptable to increases in packaging density.
In order to eliminate the aforementioned constraints of the standard leadframe, an improved leadframe has been developed at Micron Technology, Inc. The leadframe, which has a wire-bonding region at each end of the die attachment region, supports two types of die-mounting and wire bonding systems. The first system is relatively conventional, with the circuitry of the die face-up, the die adhesively and insulatedly attached by its substrate (back-bonded) to extensions of the individual leadframe leads, and the bonding wires running from bonding pads on the die, over the edge of the die, to the ends of the lead extensions below. The second system was developed by William C. Ward at IBM for his "A-wire" design leadframe, and requires that the die be adhesively and insulatedly attached by its face (face-bonded) to the leadframe, with the bonding wires running from the wire bonding pads of the die, through an aperture in the leadframe, to the ends of the lead extensions above. Micron Technology's improved leadframe is described in a patent application (now abandoned) entitled "Leadframe Having a Wire-Bonding Region at Each End of the Die Attachment Region", accorded serial number 07/381,144 and a filing date of July 17, 1989. This improved leadframe is described below, in reference to FIGS. 2 and 3.
Referring now to FIG. 2, a dual-row, inline-lead embodiment of the improved leadframe 21 has no discrete die bonding pad. A rectangular die is attached to lead extensions 22, which all line in a common plane, via an electrically-insulative, mechanically-protective film layer (not shown in this figure), both sides of which are adhesively coated. Although polyimide film is presently used, other substitute films may be identified. By definition, a lead extension 22 is continuous with its associated lead 23. The lead extension ultimately will be encapsulated within the plastic of the package body, while the lead, itself, will remain exposed. The outline of the package body 24 shows the relative position of leadframe 21 with respect to the encapsulating plastic of the package. Since this is a dual-row leadframe, there are both top row leads 23T and bottom row leads 23B. For this particular embodiment, each row of leads (23T and 23B) is divided into right and left groups (designated by the letters "R" and "L", respectively) of substantially equal number by the leadframe's axis of lateral symmetry 25. Each of the resulting four groups is further divided into two subgroups on the basis of its location with respect to left and right wire-bonding zones, which are demarcated by broken lines 26L and 26R, respectively. Those leads located between wire-bonding zones 26L and 26R pertain to an "inner" lead subgroup (designated by the letter "I"). The remainder of the leads pertain to an "outer" lead subgroup (designated by the letter "0"). For example, the six leads 23TRI pertain to the top, right, inner lead subgroup, whereas the four leads 23BLO pertain to the bottom, left, outer lead subgroup. Each lead 23*** (the asterisks indicate a reference to all leads in general) has an extension 22 which is shaped such that it is directed to a lead wire-bonding zone. All leads pertaining to the right subgroup 23*R* (here the asterisks indicate a reference to all leads within the right subgroup) have extensions 22 which terminate in wire-bonding zone 26R, located to the right of the leadframe's lateral line of symmetry 25; all leads pertaining to the left subgroup 23*L* have extensions which terminate in wire-bonding zone 26L, located to the left of the leadframe's lateral line of symmetry 25. Each wire-bonding zone consists of a group of lead extensions which are located on the perimeter of a substantially rectangular or trapezoidal aperture within said leadframe. The extensions of leads 23*LO are angled so that the ends thereof are located to the left of the aperture of left wire-bonding zone 26L, whereas the extensions of leads 23*LI are angled so that they are located to the right of left wire-bonding zone 26L. An aperture is thus formed between the ends of the extensions of leads 23*LO and those of leads 23*LI. The extensions of leads 23*R* are arranged in a configuration that is a mirror of those on the left side of the leadframe. Although the lead extensions 22 are generally L-shaped, they could also be curved or irregularly shaped. It is important only that the lead extensions 22 neither touch nor overlap one another. Hereinafter, this embodiment of the improved leadframe shall be referred to as the dual-row, inline-lead improved leadframe.
Referring now to FIG. 3, a single-row, inline-lead embodiment of the improved leadframe 31 is similar to the embodiment depicted in FIG. 2, except that the top row of leads has been eliminated. Therefore, the top and bottom lead designators "T" and "B", respectively, are not employed in reference to the leads 34 of leadframe 31. Parts of the leadframe of FIG. 3 that are equivalent to those of the leadframe of FIG. 2 are similarly numbered. Leadframe 31 has an axis of lateral symmetry 25, and left and right wire-bonding regions 26L and 26R, respectively. The design of leadframe 31 may be used in a single inline package (SIP), in a zig-zag inline package (ZIP), or in other single-row, inline package configurations. Hereinafter, this embodiment of the improved leadframe shall be referred to as the single-row, inline-lead embodiment of the improved Micron leadframe.